Tristan you have already got some really good replies to this topic in your identical thread in the RAW/Post Processing forum. It's due to posterization, quite possibly aggravated by processing an 8 bit image. The best option would be to do all of the work in you RAW processor first, and then export as a 16 bit image to finish your processing if needed.
If you made the error of only shooting the image in JPEG, well you could try processing that in ACR/Lightroom as the RAW processor will still give better results even when using a JPEG file, although not as good as using a RAW. The RAW processor works in 16 bit all the time. I also prefer using the ACR/LR channel mixers for my monochrome conversions over using the PS Channel mixer, the RAW option give you an additional colour channel over PS.
Alan